let isometry: Isometry3<f32> = nalgebra::try_convert(matrix4).expect("The matrix was not an isometry");

However this will perform expensive checks to make sure the given matrix is actually a direct isometry (that is, it does not contain any scaling or reflection). If you are sure your input matrix really represent an isometry and want to avoid those checks for performance reasons, you can use nalgebra::convert_unchecked instead.